There are numerous situations in which a tube is inserted in a tube sheet, as in a boiler, and it is highly desirable to provide close contact in the nature of an interference fit between the external surface of the tube and the surface of the bore. If a crevice remains surrounding the tube, it may serve as a starting point for destructive corrosion.
A tube sheet structure in which particularly high standards must be met and severe corrosion problems may be encountered is found in the heat exchangers of nuclear power plants. In this environment, the tube sheet is in the form of a steel plate several feet thick through which bores extend transversely to receive tubes. The interfaces between the tube sheet and the tubes on the secondary side of the tube sheet are exposed to water, extremely high temperature and changing pressure. If crevices remain or are formed later between the tubes and the tube sheet, a substantially increased possibility exists for corrosive action, requiring a reduction in the estimated minimum life-expectancy of the structure. It is, therefore, crucial that any such crevice formation be minimized or eliminated in a highly predictable and repeatable manner. Moreover, a technique that requires a minimum of time is desirable because upward of a thousand tubes may pass through a single tube sheet.
It is known to seal the outer surface of a tube against the surrounding surface of a bore by applying hydraulic pressure to the interior of the tube, thus expanding the tube. Sufficiently high pressure is applied to expand the bore radially by elastically deforming the tube sheet while plastically deforming the tube. When the pressure is removed, the bore contracts and compresses the tube. The pressure is applied by a mandrel that is inserted in the tube. Axially spaced seals encircle the mandrel to define a pressure zone between them within which this radial expansion takes place.
A critical area in which it is difficult to obtain the desired controlled expansion of the tube is at the end of the bore at the secondary side of the tube sheet. If the seal at that end of the mandrel is positioned slightly beyond the surface of the tube sheet, an annular bulge will be created when the pressure is applied, since radial expansion forces will exist in a portion of the tube that protrudes from the tube sheet and is not confined within the bore. This bulge will act as a stress riser and will significantly reduce the strength and integrity of the tube.
To avoid the formation of such secondary end bulges, it has been necessary to position the inner seal of the mandrel so that it is within the bore, spaced from the secondary surface of the tube sheet. While this technique solves the problem of bulge formation, it often allows a small crevice to remain on the secondary side of the tube sheet at precisely the location exposed to the most intensive corrosive action.
It is an objective of the present invention to provide an improved method for installing tubes that permits the elimination or substantial reduction of any such crevice without resulting in the formation of a bulge.